The present invention relates generally to piezoelectric effect resonators, and particularly to temperature control of the piezoelectric effect crystal element of such resonators.
U.S. Pat. No. 3,715,563 issued on Feb. 6, 1973 in the name of Martin Bloch, one of the inventors herein, to the assignee of this application discloses a piezoelectric device which includes a piezoelectric effect crystal element disposed in an evacuated chamber. As described in the U.S. Pat. No. 3,715,563, piezoelectric devices such as quartz crystal devices are known to be temperature sensitive and were traditionally placed in high tolerance ovens which maintained the temperature of the devices to a high degree of accuracy. The U.S. Pat. No. 3,715,563 discloses that rather than use conventional ovens, thin films deposited directly on the piezoelectric effect crystal elements can be utilized as electric resistance contact heaters to produce substantially improved results.
In devices of the type disclosed in the U.S. Pat. No. 3,715,563, an electric field is generated in a piezoelectric effect crystal element by means of an exciting signal applied to the crystal element by electrodes disposed opposite each other on both sides of the crystal element. The electrodes are disposed inwardly of the edge of the crystal element and lead contacts are disposed on the crystal element surface extending from a respective electrode outwardly to the edge of the crystal element. Such an arrangement of electrodes generates an electric field directed normal to a major surface of the crystal element, i.e., along the thickness of the crystal element, and is referred to as a "thickness excitation field" or simply a "thickness field". In order to uniformly apply heat to a crystal element having the electrode arrangement described above, the U.S. Pat. No. 3,715,563 discloses an arrangement of a contact heater, electrodes and electrode lead contacts in which the electrode lead contacts on opposed sides of the crystal element are offset and annular contact heater elements are applied to both sides of the crystal element overlapping with each other except in the regions occupied by the lead contacts. As a result of this arrangement, a constant temperature is established in the crystal element even in regions of the crystal element covered by the electrodes and lead contacts, to compensate for varying ambient temperatures.
Recent research has revealed the desirability of generating an electric field in piezoelectric effect crystal elements parallel to a major surface of the crystal element, such a field being referred to as a "lateral excitation field" or simply as a "lateral field". See, for example, U.S. patent application Ser. No. 738,697 filed May 28, 1985. The disclosures of U.S. Pat. No. 3,715,563 and U.S. patent application Ser. No. 738,697 are incorporated herein by reference.
Lateral fields may be generated in piezoelectric effect crystal elements by arranging the electrodes on the same side of the crystal element, as disclosed, for example, in the patent application Ser. No. 738,697. Specifically, the patent application Ser. No. 738,697 discloses the desirability of utilizing an SC cut quartz crystal element with electrodes disposed on the same side of the crystal element for generating a lateral field therein. Although the U.S. Pat. No. 3,715,563 discloses that improved temperature stabilization of a piezoelectric effect device can be realized with a contact heater deposited directly on the piezoelectric effect crystal element, a heater pattern such as the one disclosed in the U.S. Pat. No. 3,715,563 is quite difficult to apply to a lateral field excited crystal element because of the limited space available on the side of the crystal element having the electrodes and also because of the desire to make available portions of the crystal element for adjusting the frequency at which the crystal element vibrates. Thus, applying resistive heater portions to both sides of a crystal element in order to obtain a uniform temperature gradient throughout the crystal element, as disclosed in the U.S. Pat. No. 3,715,563, presented a problem with respect to lateral field excited crystal elements in configuring and locating the heater elements and the electrodes, and connecting leads and supports to the crystal element.